Discovery of a second kHz QPO peak in 4U 1608-52

B.A. Vaughan (Caltech); D. Psaltis (Harvard/CfA); E. Kuulkers (University of Oxford); F. K. Lamb (University of Illinois); J. van Paradijs (University of Amsterdam); M. Mendez; M. van der Klis; W.H.G. Lewin (MIT); W. Zhang (NASA/GSFC)

Discovery of a second kHz QPO peak in 4U 1608-52

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arxiv astro-ph/9712085 v1 pith:MZEK3LFM submitted 1997-12-05 astro-ph

Discovery of a second kHz QPO peak in 4U 1608-52

M. Mendez , M. van der Klis , J. van Paradijs (University of Amsterdam) , W.H.G. Lewin (MIT) , B.A. Vaughan (Caltech) , E. Kuulkers (University of Oxford) , W. Zhang (NASA/GSFC) , F. K. Lamb (University of Illinois)

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D. Psaltis (Harvard/CfA)

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Using a new technique to improve the sensitivity to weak Quasi-Periodic Oscillations (QPO) we discovered a new QPO peak at about 1100 Hz in the March 1996 outburst observations of 4U 1608-52, simultaneous with the ~ 600-900 Hz peak previously reported from these data. The frequency separation between the upper and the lower QPO peak varied significantly from $232.7 \pm 11.5$ Hz on March 3, to $293.1 \pm 6.6$ Hz on March 6. This is the first case of a variable kHz peak separation in an atoll source. We discuss to what extent this result could be accommodated in beat-frequency models such as proposed for the kHz QPOs. We measured the rms fractional amplitude of both QPOs as a function of energy, and we found that the relation is steeper for the lower than for the upper frequency peak. This is the first source where such difference between the energy spectrum of the two kHz QPOs could be measured.

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